Arsenic ivith Ccesium and Rubidium. 89 



compounds of arsenious oxide with potassium halides, but this 

 class of bodies has been most thoroughly studied by Eriidorff.'* 

 His results indicate the existence of two types of this class of 

 compounds, the first containing one molecule of alkaline 

 halide to one of arsenious oxide, the other having these con- 

 stituents in the ratio 1:2. In the present investigation a com- 

 plete series of the caesium and rubidium oxyhalides of the 1 : 1 

 type was obtained, while the formation of the 1 : 2 type was 

 not observed. There is evidently a gradation in stability from 

 the oxychlorides to the oxyiodides, the stability increasing 

 with increasing atomic weight of the halogens. Attempts to 

 prepare double halides of pentavalent arsenic were without 

 success. 



The new compounds to be described in this article are the 

 following : 



3CsCl . 2AsC! 3 3CsBr. 2AsBr 3 3CsI. 2AsI 8 



3RbCl . 2AsCl 3 3RbBr. 2AsBr 3 3RbI . 2AsI 3 



CsCl.As.O, CsBr.As.O, CsT . As 2 3 



RbCl . As 2 3 RbBr . As~ 2 3 Rbl . As 2 3 



The compound As 2 3 . KI which had not been observed by 

 Rudorfff was obtained in attempts to prepare a double iodide 

 of potassium and arsenic. 



It is to be noticed that only one type of double halides was 

 obtained. This type corresponds to the most readily prepared 

 double chloride of caesium with antimony, described by Rem- 

 sen and Saunders % and of caesium and bismuth, described by 

 Remsen and Brigham.§ Many attempts were made to pre- 

 pare arsenic double halides of other types than this single one, 

 but these have invariably been without success, although sev- 

 eral types of antimony and bismuth double halides have been 

 described. 



It has been shown in several instances by Wells and Wheeler || 

 that caesium and rubidium halides form more stable or more 

 complete series of double salts than the halides of the other 

 alkali-metals. This fact is again well illustrated in the double 

 halides of arsenic, for the caesium and rubidium double halides 

 are prepared without difficulty, while with potassium no double 

 halides were obtained. 



Methods of preparation. — To prepare the double halides, a 

 strong acid solution is necessary, in order to prevent the 

 decomposition of the arsenious halide and the consequent 

 formation of oxy-com pounds. The double halides are less 



*Ber d. Deutsch. Chem. Ges . xix. 2668; ibid. xxi. 3053. 

 + Loc. cit. % Am. Chem. Jour., xiv, 152. 



§ Ibid., p. 161. || This Journal, III, xliii. 475; TJI, xliv, 42. 



